Japanese Patent No. 3,012,417 describes a power transmission device for a 2WD (two-wheel drive) motorcycle. This power transmission device is provided with two housings. One housing is fixed on a body frame whereas the other housing is assembled with a front-wheel fork. These two housings are jointed so all-around rotatably that the front wheel can be turned 360 degrees by manipulating a handle.
The housing on the body side is provided with an input shaft whereas the housing on the front wheel side is provided with an output shaft. Between the input shaft and the output shaft, there are arranged two reversely rotating transmission lines and a differential gear mechanism. This differential gear mechanism is enabled to absorb such a rotating speed difference between the two lines as is caused as the front wheel side housing turns, thereby to prevent the mutual interference between the drive force for rotation by the engine and the drive force for turning by the handle.